Giant ragweed (Ambrosia trifida), used in herbal medicine as an antiseptic and to treat intestinal worms and fever, contains significant quantities (ca. 0.003% by weight) of 3-(3-buten-1-ynyl)-6-(1,3-pentadiynyl)-1,2-dithiin (thiarubrine B), and lesser quantities of 3-(5 hexen-1,3-diynyl)-6-(1-propynyl)-1,2-dithiin (thiarubrine A) and related compounds. Thiarubrines are notable as a class of light-sensitive, biologically potent, structurally unique polyacetylenic plant pigments containing an antiaromatic 8.pi.-electron 1,2-dithiin ring system. First identified in 1964-65 in species of Compositae (Asteraceae) used for skin infections and intestinal parasites by native people in Africa and Canada, thiarubrines show good light-mediated activity against human immunodeficiency virus (HIV-1) and possess significant antibiotic, antiviral and nematicidal activity both in the light and in the dark. Ten 3,6-disubstituted 1,2-dithiins have so far been identified in nature, including some with epoxide, alcohol, or chloro groups, as well as a 1,2-dithiin 1-oxide. ##STR1## In addition a monosubstituted (at ring position 3) 1,2-dithiin has also been identified from a North African medicinal plant.
Two groups have reported syntheses of simple 1,2-dithiins that could be considered models for thiarubrines, but in the 30 years since their discovery, no chemical syntheses of the natural products have been reported. The absence of a flexible chemical synthesis hampers detailed study of the biological activity and chemical and physical properties of thiarubrines. There is therefore a need for a chemical synthesis of 1,2-dithiins that allows wide flexibility in the substituents at positions 3 and 6.